72 PHYSIOLOGY. 



70, 71.) The same phenomenon has been observed in manj' plants. Light 

 then has an iniluence on chlorophyll bodies, tf) some extent determining 

 their position. In weak light they are arranged so that the flattened sur- 

 faces are exposed to the incidence of the rays of light, so that the chloro- 

 phyll will absorb as great an amount as possible of kinetic energy; but 

 intense light is stronger than necessary, and the chlorophyll bodies move so 

 that their edges are exposed to the incidence of the rays. This movement 

 of the chlorophyll bodies is different from that which takes place in some 

 water plants like elodea. The chlorophyll bodies in clodea are free in the 

 protoplasm. The protoplasm in the cells of elodea streams around the 

 inside of the cell wall much as it does in nitella and the chlorophyll bodies 

 are carried along in the currents, while in nitella they are stationary. 



